The present invention relates to a method for salvaging aromatic polymeric values, such as polycarbonate blend values from painted aromatic polycarbonate blend surfaces. More particularly, the present invention relates to the use of an aqueous bath containing an organic solvent, an alkali metal hydroxide, or alkaline earth metal hydroxide and an effective amount of a surfactant to effect the removal of thermally cured polyurethane paint from the surface of a molded blend of aromatic polycarbonate and aromatic polyester.
There is shown by Bussink et al, U.S. Pat. No. 4,267,096, molded blends of aromatic polycarbonate, amorphous aromatic polyester and a hydrogenated block copolymer of a vinyl aromatic compound and a diolefin. These molded blends exhibit high impact strength and resistance to melt flow, environmental stress crazing and crack resistance. The use of molded blends of aromatic polycarbonate and aromatic polyester in high impact applications, such as automobile bumpers, are shown by Van Abeelen et al, U.S. Pat. No. 4,568,712 and Lohmeijer et al, U.S. Pat. No. 4,654,400. After molding, the plastic bumper is often painted with a thermally curable polyurethane paint.
In most instances, used painted bumpers are discarded. These rejected articles cannot be readily recycled because it has been found difficult to remove the thermally cured polyurethane paint. Attempts to extrude such paint containing thermoplastic material which has been granulated, can result in the decomposition of the polyurethane paint to form hexamethylene diisocyanate, a toxic compound. Extrusion in the presence of the polyurethane paint also can adversely affect the performance of any resulting remolded part, since paint residues can concentrate stress and diminish physical properties.
Mechanical methods for the direct removal of paint from thermoplastic surfaces, such as by sandblasting, or with pressurized CO.sub.2 have been shown to be environmentally and economically unattractive. In copending application Ser. No. 07/981,668, filed Nov. 25, 1992, a method is described for removing silicone hardcoats from polycarbonate substrates utilizing a 50% aqueous caustic solution in combination with isopropanol. Efforts to use this procedure for painted thermoplastic bumpers have been found to result in only the partial removal of thermally cured polyurethane paint from the surface of painted aromatic polycarbonate parts.